Method for improving spatial perception in virtual surround

ABSTRACT

A method for improving the spatial perception of multiple sound channels when reproduced by two loudspeakers, generally front-located with respect to listeners, each channel representing a direction, applies some of the channels, such as sound channels representing directions other than front directions, to the loudspeakers with headphone and crosstalk cancelling processing, and applies the other ones of the sound channels, such as sound channels representing front directions to the loudspeakers without headphone and crosstalk cancelling processing. The headphone processing includes applying directional HRTFs to channels applied to the loudspeakers with headphone and crosstalk cancelling processing and may also include adding simulated reflections and/or artificial ambience to channels applied to the loudspeakers with headphone and crosstalk cancelling processing.

TECHNICAL FIELD

The invention relates to audio signal processing. More particularly, theinvention relates to improving the spatial perception of a multichannelsound source when reproduced by two loudspeakers.

BACKGROUND ART

Multichannel sound reproduction systems such as Dolby Pro Logic or DolbyDigital (Dolby, Dolby Pro Logic and Dolby Digital are trademarks ofDolby Laboratories Licensing Corporation) require, for example, fivespeakers, placed at particular locations and particular angles. This canbe costly and space consuming. It would be desirable to have surroundsound without rear loudspeakers, to save on cost and space. However,conventionally, front loudspeakers only provide front sound images.

It is known to process multiple channels representing sounds from manydirections, and combine them into two signals for reproduction overheadphones, retaining the apparent multiple directions. With headphonereproduction the left signal goes to the left ear, and the right to theright, with no crosstalk. Sounds can appear to come from the sides ofthe listener as well as from the front, or in some cases the rear.

Considering each of the multichannel inputs as representing sound from aparticular direction, such processing for headphones typically includesat least applying appropriate HRTFs (head related transfer functions) toeach input to simulate the paths from its desired apparent direction tothe two ears, so that the headphone listener perceives each channel ascoming from the desired direction. Such headphone processors, whichprovide two outputs in response to more than two inputs, are referred toby a variety of names such as “multi-axis binaural steering” processors,“multi-channel binaural synthesizers”, “headphone virtual surround”processors, and the like. Some headphone processors also provideprocessing in addition to applying directional HRTFs, such as addingsimulated reflections and/or artificial ambience to one or more of thechannels. All such processors, whether employing only directional HRTFsor also additional processing, such as artificial reflections and/orambience, are referred to herein as “headphone processors.” Someexamples of headphone processors include those described in publishedInternational Application WO 99/14983 (designating the United States)and in U.S. Pat. Nos. 5,371,799; 5,809,149; and 6,195,434 B1. Each ofsaid application and patents are hereby incorporated by reference, eachin their entirety.

Conventional two-channel stereophonic material is intended forreproduction over two loudspeakers. Each of the listener's ears receivessound from both loudspeakers, with, of course, different path lengthsand frequency responses. In other words, there is acoustic crosstalk. Ingeneral, all sounds so reproduced appear to lie within the space betweenthe loudspeakers.

It is also known to modify signals prior to application to twoloudspeakers to cancel the acoustic crosstalk, at least partially. Thisallows the apparent position of sounds to lie well outside the spacebetween the loudspeakers, and is the basis of “virtual surround”processes. To the extent that the crosstalk is cancelled, the soundsentering the ears from the two loudspeakers resemble those provided byheadphones, i.e., without crosstalk. Crosstalk cancellers (sometimesreferred to as “spatializers” or “panoramic processors”) are well knownin the art, dating at least from U.S. Pat. No. 3,236,949 (Atal andSchroeder), which patent is hereby incorporated by reference in itsentirety. A computer-software-implemented acoustic-crossfeed cancellerusing very low processing resources of a personal computer is disclosedin U.S. patent application Ser. No. 08/819,582 of Davis et al, filedMar. 14, 1997, which application is hereby incorporated by reference inits entirety.

As is also known, signals representing multiple channels, includingsounds originally coming from outside the space between the loudspeakerscan be processed as if for reproduction over headphones and then fed viaan acoustic crosstalk canceller to two front loudspeakers arranged in aconventional stereo configuration, such as at the sides of a computermonitor or a television picture tube. This combination of headphoneprocessing and crosstalk cancellation allows the apparent position ofsound sources to lie to the sides, or in some cases the rear, using onlya pair of front loudspeakers.

FIG. 1 is a schematic block diagram showing a prior art arrangement inwhich the multiple channels of a multichannel source, such as afive-channel source (each channel representing a direction, such as leftfront, center front, right front, left surround and right surround), areapplied to a headphone processor 2. The two outputs of the headphoneprocessor are applied to a crosstalk canceller 4, which also has twooutputs. One output of the crosstalk canceller is applied to a firstloudspeaker 6 and the other output is applied to a second loudspeaker 8.

The combination of headphone processing and crosstalk cancellationfeeding a pair of loudspeakers is superior to a crosstalk cancelleralone because the processing for headphone reproduction introducesadditional directional cues by introducing directional HRTFs (crosstalkcancellers may include only “one ear to the other” HRTFs) and, in someheadphone processors, simulated multiple acoustic paths (includingreflections) between apparent image positions (outside the loudspeakers)and the listener's ears. Thus, with combined headphone processing andcrosstalk cancellation, virtual sound images may appear not only at thesides of a listener's head but also from further back.

However, there are disadvantages of such a combined headphone processingand crosstalk cancellation scheme. The front sound channels deft front,center front, right front) of the multichannel source are intended to bereproduced over loudspeakers and are satisfactorily reproduced by twoloudspeakers that reproduce the left front and right front channels andalso provide a virtual or “phantom” center front image (provided, ofcourse, that the listener is appropriately located with respect to thetwo loudspeakers). Consequently, processing the front sound channels isnot necessary and should be avoided (in accordance with the “leasttreatment” principle). Headphone processing of the front channelsinvolves at least the application of directional HRTFs that may causecolorations or changes in timbre, for example. Other headphoneprocessing techniques, for example the simulation of reflections orreverberation, may introduce other noticeable and unnecessaryalterations of the front channel signals or may produce artifacts.Crosstalk cancellation may also adversely affect the front channels.Crosstalk cancellation is most effective when the playback environment,the listening room, introduces little by way of reflections.Consequently, in practical “real listening room” applications, crosstalkcancellation is incomplete. Thus, even if headphone processing of thefront channels were transparent, the subsequent crosstalk cancellationin prior art of the type shown in FIG. 1 would likely impair thereproduced front channel sound.

In accordance with the present invention, impairment of the frontchannel reproduction is avoided while retaining the benefits of improvedsurround channel reproduction from a pair of loudspeakers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram showing a prior art arrangement inwhich the multiple channels of a multichannel source, such as afive-channel source (each channel representing a direction, such as leftfront, center front, right front, left surround and right surround), areapplied to a pair of front-located loudspeakers via a headphoneprocessor and a crosstalk canceller.

FIG. 2 is an idealized functional block diagram of an arrangement inaccordance with the present invention.

FIG. 3 is an idealized functional block diagram of an arrangement inaccordance with the present invention in which the headphone processingapplied to some audio channels includes adding simulated reflectionsand/or artificial ambience and other audio channels are withoutheadphone and crosstalk cancelling processing and without addingsimulated reflections and/or artificial ambience.

DISCLOSURE OF THE INVENTION DETAILED DESCRIPTION OF THE PREFERREDEMBODIMENTS

FIG. 2 shows an idealized functional block diagram of an arrangement inaccordance with the present invention that receives the multiplechannels of a multichannel source, such as a five-channel source (eachchannel representing a direction, such as left front (L), center front(C), right front (R), left surround (Ls) and right surround (Ls)),applies the secondary channels (such as left surround and rightsurround) via a headphone processor and a crosstalk canceller to a pairof front-located loudspeakers and applies the main channels (such asleft, center, right) to the pair of front-located loudspeakers withoutheadphone or crosstalk cancelling processing.

The source of the multidirectional sound sources applied to thearrangement of FIG. 2 is not critical and may be any suitable source,including, for example, a Dolby Pro Logic source, a Dolby Digitalsource, a Digital Theater Systems Corporation (“DTS”) source (“DTS” is atrademark), a discrete source, or some other source. Although theinvention will be described in connection with an embodiment that hasthree main channels and two secondary channels, the invention is not solimited. For example, there may be only two main channels, such as leftand right, and/or there may be more than two secondary channels, such asfive secondary channels (for example, left front surround (LFS), leftrear surround (LRS), right front surround (RFS), right rear surround(RRS) and center surround (CS)). The number of secondary channels islimited only by the complexity of the headphone processor and itsability to simulate the placement of sounds in a large number ofdirections.

As shown in FIG. 2, a portion of the arrangement is a conventional priorart Dolby MP Matrix encoder configured as a 3:2 encoder. The matrixencoder 10 accepts three separate input signals; left front, centerfront, and right front (L, C, R), and creates two final outputs, lefttotal and right total (Lt and Rt). The C input is divided equally andsummed with the L and R inputs with a 3 dB level reduction in order tomaintain constant acoustic power.

The left-total (Lt) and right-total (Rt) encoded signals may beexpressed asLt=L+0.707C; andRt=R+0.707C,where L is the left front input signal, R is the right front inputsignal, and C is the center front input signal. When the Lt encodedsignal is reproduced by a left-located front loudspeaker and the Ltencoded signal is reproduced by a right-located front loudspeaker, avirtual or “phantom” center channel image may be perceived by a properlylocated listener. The use of a center channel is not critical and may beomitted, in which case the L and R input signals may be coupled directlyto the loudspeakers without any requirement for a matrix to mix in thecenter channel. If an encoder matrix is employed, it need not mix in thecenter channel at −3 dB but may employ some other mixing level. In anycase, in accordance with the present invention, the main channelsintended for reproduction by two front-positioned loudspeakers (such asthe left front, center front (if employed) and right front channels) arenot applied to the two loudspeakers via a headphone processor and/or acrosstalk canceller.

Still referring to FIG. 2, the left surround (Ls) and right surround(Rs) supplemental channel signals are applied to the left surround (Ls)and right surround (Rs) inputs of a headphone processor 12. Headphoneprocessor 12 has characteristics such as described above. Such headphoneprocessors may also have inputs for left front (L), center front (C) andright front (R) signals, as shown in FIG. 2; however, those inputs arenot used. As explained above, there may be additional supplementalchannel signals applied to headphone processor 12 provided that thedevice is capable of processing more than two secondary channel inputs.Headphone processor 12 provides two output signals, left headphone (Lh)and right headphone (Rh). These outputs are intended to provide aheadphone listener with the perception that each of the secondarychannel inputs is coming from the desired direction. The Lh and Rhoutput signals are not applied to headphones but to a crosstalkcanceller 14 that, in turn, provides crosstalk cancelled versions of theLh and Rh signals, designated here as left canceller (Lc) and rightcanceller (Rc). The Lc signal is additively combined with the Lt signalin a summer 16 to produce a left virtual (Lv) and the Rc signal isadditively combined with the Rt signal in a summer 18 to produce a rightvirtual (Rv) signal. The Lv signal may then be coupled to a suitableleft-positioned front-located loudspeaker (not shown) and the Rv signalmay then be coupled to a suitable right-positioned front-locatedloudspeaker (not shown). Reproduction of the Lv and Rv signals by suchloudspeakers provides a properly located listener with the perception ofmain channel sounds without the shortcomings of headphone processorand/or crosstalk canceller processing while providing enhanced phantomimages of the secondary channel sounds.

FIG. 3 is generally the same as FIG. 2 except that instead of aHeadphone Processor 12, a block 12′ applies headphone processingincluding adding simulated reflections and/or artificial ambience. Suchprocessing is not applied to Lt and Rt, thus the figure shows that Ltand Rt are without headphone and crosstalk cancelling processing andwithout adding simulated reflections and/or artificial ambience.

It should be understood that implementation of other variations andmodifications of the invention and its various aspects will be apparentto those skilled in the art, and that the invention is not limited bythese specific embodiments described.

The present invention and its various aspects may be implemented inhardware, or as software functions performed in digital signalprocessors, programmed general-purpose digital computers, and/or specialpurpose digital computers, or as a combination of hardware and softwarefunctions. Interfaces between analog and digital signal streams may beperformed in appropriate hardware and/or as functions in software and/orfirmware.

The invention claimed is:
 1. A method for improving the spatialperception of multiple sound channels when reproduced by twoloudspeakers, each channel representing a direction, comprising applyingsome of said channels to said loudspeakers with headphone and crosstalkcancelling processing, wherein said headphone processing includes addingsimulated reflections and/or artificial ambience to said some of saidchannels, and applying the other ones of said sound channels to saidloudspeakers without headphone and crosstalk cancelling processing andwithout adding simulated reflections and/or artificial ambience to suchother ones of said sound channels.
 2. A method according to claim 1wherein said two loudspeakers are generally front-located with respectto listeners and wherein sound channels representing directions otherthan front directions are applied to said loudspeakers with headphoneand crosstalk cancelling processing and sound channels representingfront directions are applied to loudspeakers without headphone andcrosstalk cancelling processing.
 3. A method according to claim 2wherein said headphone processing further includes applying directionalHRTFs to channels applied to said loudspeakers with headphone andcrosstalk cancelling processing.
 4. A method according to any one ofclaims 1-3 wherein applying sound channels to said loudspeakers withoutheadphone and crosstalk cancelling processing includes encoding suchsound channels to reduce the number of such sound channels to two whenthere are more than two of such sound channels.
 5. A method according toclaim 4 wherein said encoding comprises matrix encoding.
 6. A methodaccording to claim 5 wherein said matrix encoding is 3:2 matrixencoding.
 7. Audio apparatus for improving the spatial perception ofmultiple sound channels when reproduced by two loudspeakers, eachchannel representing a direction, comprising a processor receiving someof said sound channels and delivering two output signals, said processorincluding a headphone processor employing directional HRTFs and acrosstalk canceller, wherein said headphone processor further includes asimulated reflections and/or artificial ambience processor, a firstadditive combiner receiving one of the outputs of said processor andreceiving the channels other than the channels applied to said processorwith relative proportions in accordance with their directions, whereinthe channels other than the channels applied to said processor have noadded simulated reflections and/or artificial ambience, and providing asignal for one of said loudspeakers, a second additive combinerreceiving the other of the outputs of said processor and receiving thechannels other than the channels applied to said processor with relativeproportions in accordance with their directions, wherein the channelsother than the channels applied to said processor have no addedsimulated reflections and/or artificial ambience, and providing a signalfor the other of said loudspeakers.
 8. The apparatus of claim 7 whereinsaid two loudspeakers are generally front-located with respect tolisteners and wherein sound channels representing front directions arecoupled to the first and second additive combiners and sound channelsrepresenting directions other than front directions are coupled to saidheadphone processor.
 9. The apparatus according to claim 7 or claim 8further comprising an N:2 matrix encoder, wherein ones of the multiplesound channels not coupled to the headphone processor are coupled tosaid additive combiners via the N:2 encoder.
 10. The apparatus accordingto claim 7 wherein there are five sound channels, L, C, R, Ls, and Rs,said processor receiving said Ls and Rs signals, said L, C, and Rchannels applied to said first and second additive combiners withrelative proportions such that all of the L channel and none of the Rchannel is received by one of the combiners, all of the R channel andnone of the L channel is received by the other of the combiners, and asubstantially equal proportion of the C channel is received by each ofthe combiners.
 11. A method for improving the spatial perception ofmultiple sound channels when reproduced by two loudspeakers, eachchannel representing a direction, comprising applying some of saidchannels to said loudspeakers with headphone and crosstalk cancellingprocessing, wherein said headphone processing includes adding simulatedreflections and/or artificial ambience to channels applied to saidloudspeakers, and applying the other ones of said sound channels to saidloudspeakers without headphone and crosstalk cancelling processing andwithout adding simulated reflections and artificial ambience to suchother ones of said sound channels.
 12. A method for improving thespatial perception of multiple sound channels when reproduced by twoloudspeakers, each channel representing a direction, comprising applyingsome of said channels to said loudspeakers with headphone and crosstalkcancelling processing, wherein said headphone processing includes addingsimulated reflections and artificial ambience to said some of saidchannels, and applying the other ones of said sound channels to saidloudspeakers without headphone and crosstalk cancelling processing andwithout adding simulated reflections and artificial ambience to suchother ones of said sound channels.